1. Field of the Invention
The present invention relates to a color cathode ray tube and, more particularly to a color cathode ray tube having a panel with a high wedge ratio, wherein a shadow mask or a screen is designed to an optimum state, or the light absorption coefficient of the panel is set to an optimum value, thereby having improved bright uniformity (BU) characteristics.
2. Description of the Conventional Art
Generally, a color cathode ray tube is a display widely used in a television receiver, or the monitors of oscilloscope for measurement and RADAR for observation. The color cathode ray tube displays an image on a display through Red, Green and Blue fluorescent materials which are excited by the electron beams from the electron guns (or cathodes).
FIG. 1 is a cross-sectional view of a shadow mask-type color cathode ray tube, wherein a rectangular shape panel 1, a funnel 2 connected to the panel 1, a cylindrical glass neck 3 connected to a smaller-diameter end of the funnel 2, and an in-line electron gun 4 within the neck 3 are illustrated.
A screen 5 coated with a fluorescent material is installed on an inner surface of the panel 1, a shadow mask 6 for selecting color on the screen 5 is installed at a predetermined distance from the screen 5, and a deflection yoke 7 which generates a pin cushion type horizontal deflection magnetic field and a barrel type vertical deflection magnetic field is mounted on outer surfaces of the neck 3 of the funnel 2.
FIG. 2 is a detailed view of the screen 5. The screen includes fluorescent dots 5a coated with red, green, and blue(RGB) fluorescent materials on an inner effective surface of the panel 1 and a black matrix layer 5b filled with a black coating material on the regions except for the fluorescent dots 5a. Herein, the RGB fluorescent materials are divided into a dot type and a stripe type according to the shape of the fluorescent materials formed on the screen.
FIG. 3 is a detailed view of the shadow mask 6. The shadow mask includes a plurality of slots (or holes) 6a corresponding to the fluorescent dots 5a, in order that electron beams emitted from the electronic gun may pass through the shadow mask 6 and be incident upon the fluorescent dots 5a, and the slots are coated with a spray after blackening.
When electron beams 8 are emitted from the electron guns 4, the electron beams 8 are deflected by horizontal and vertical deflection fields of the deflection yoke 7 in order to scan the entire screen, and then the deflected beams 8 converge on the plurality of slots 6a formed on the shadow mask 6.
When the electron beams 8 having passed through each of the slots 6a are emitted on the screen 5, the RGB fluorescent materials 5c are illuminated to thereby reproduce a color image on the panel 1.
Currently, the outer surface of a panel 1 is being made flat. That is, the panel of a color cathode ray tube having curvatures of both inner and outer surfaces as shown in FIG. 4A is being changed to a panel of a color cathode ray tube having a curved inner surface and a flat outer surface, respectively as shown in FIG. 5A. The wedge ratio of the thickness of a central panel to a corner panel is usually from 150% to 250%.
However, when only the outer surface of a panel is flat as shown in FIG. 5A under the condition that both inner and outer surfaces of the panel have curvatures as shown in FIG. 4B, the uniformity of the brightness, i.e., the BU characteristics (which is one of the most important screen characteristics) decreases. A comparison between FIG. 4B and FIG. 5B shows when the outer surface of the panel is flat, the brightness characteristics has a Gauss distribution.
Hence, in order to prevent deterioration of the uniformity of the brightness, a panel with improved transmittance is fabricated by using a clear glass with a smaller light absorption coefficient and by determining the transmittances of a shadow mask and a screen by a simulation experiment design even though there is another method of improving the transmittance of the panel. Generally, the absorption coefficient of a panel is more than 0.01298.
The transmittance of a shadow mask depends on the slot area (referring to the slot width as the same meaning of the slot area), for example, the slot width at the corner portion can be 200 xcexcm and the slot width at the center portion can be 180 xcexcm. Similarly, the screen""s transmittance depends on the dot (or stripe) areas forming on the screen, which will be referred to the width of the dot as below, for example, the dot width at the corner portion can be 160 xcexcm and the dot width at the center portion can be 150 xcexcm.
In order to prevent deterioration of the BU characteristics of a flat outer surface panel, the slot widths at the corner portion and at the center portion of a shadow mask and the dot diameters at the corner portion and at the center portion of a screen are determined by the above values, and the panel is fabricated from a clear glass with a smaller light absorption coefficient by means of a simulation experiment design, thereby obtaining a predetermined light absorption coefficient and transmittances of a shadow mask and screen. However, there is a problem that the BU of the panel cannot be obtained if the wedge ratio of a panel is more than 170%, and furthermore the weight and cost of the color cathode ray tube increase because of using clear glass.
It is an object of the present invention to provide a cathode ray tube having improved bright uniformity characteristics by optimally setting the light absorption coefficient of a panel or designing the slot widths of a shadow mask or the dot diameters of a screen to an optimum state even though the panel""s wedge ratio is more than 170%.
To this end, there is provided a color cathode ray tube including a panel with a wedge ratio of more than 170%; a screen onto which electronic beams are stroked and on which fluorescent dots coated with RGB fluorescent materials and a black matrix layer filled with a black coating material throughout all the regions except for the fluorescent dots are formed; and a shadow mask on which a plurality of slots are arranged corresponding to the fluorescent dots, wherein the dot diameter at the corner portion of a screen is larger than that at the center portion by about 100xcx9c127%, and the slot width at the corner portion of a shadow mask is larger than that at the center portion by about 105xcx9c133%,